Technical Field
Embodiments described herein relates to an above-ground low-temperature tank.
Description of Related Art
In the related art, for example, as a low-temperature tank which stores low-temperature liquefied gas such as Liquefied Natural Gas (LNG), a full containment double-shell tank is used which includes a metal inner tank which stores low-temperature liquefied gas and an outer tank which includes a concrete outer wall surrounding the inner tank. In the low-temperature tank, in order to discharge the stored liquefied gas to the outside, a liquid discharging pump is installed in a pump well in the tank.
The liquid discharging pump is accommodated in a lower end of a well which penetrates a roof in the tank, and is configured to suction the liquefied gas from the lower end of the well.
In this pump, for example, if a height of a liquid surface is positioned 1 m to 3 m above a suction port, swirl and/or bubbles interfere with the suction of the pump. Accordingly, the pump cannot discharge the liquid.
Therefore, liquid corresponding to a height, which is obtained by adding a height from a bottom surface to the suction port to the height from the suction port to the liquid surface, cannot be discharged from the tank. As a result, an effective storage amount of the tank is greatly decreased.
In order to cope with this, for example, it is considered that a pump pit for a tank is formed. The pump pit for a tank is suggested in a under-ground and membrane type low-temperature tank (for example, refer to Patent Document 1, Patent Document 2, and Patent Document 3).
In the under-ground tank, since a portion protruding from the ground is small, limitation with respect to a height (depth) of a tank is decreased. Accordingly, by sufficiently increasing a depth of a hole for burying a tank, it is possible to construct a cold insulation layer, which is disposed between a foundation slab and an inner tank, with a sufficient thickness. On the other hand, in an above-ground tank, due to scenery or the like, the height of a tank is limited. Accordingly, in order to sufficiently secure an effective storage amount of a tank, a thickness of a cold insulation layer disposed between a foundation slab and an inner tank is limited.
[Patent Document 1] Japanese Patent No. 3978892
[Patent Document 2] Japanese Patent No. 4014743
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2003-278998
As described above, in the above-ground tank, particularly, the thickness of the cold insulation layer disposed between the foundation slab and the inner tank is limited. Accordingly, it is difficult to provide the pump pit for a tank.
That is, when the pump pit is provided in the above-ground tank, it is assumed that a hole portion is formed on a bottom surface of the tank, and a metal cylindrical bottomed pit main body is provided in the hole portion. However, particularly, a linear load tends to concentrate on a corner portion of a bottom portion in the pit main body. Accordingly, the linear load is transmitted to a portion of a bottom surface of the tank which receives the bottom portion of the pit main body, and large stress occurs. Since the cold insulation layer is formed of cellular glass or the like, cracks or the like occur on the cold insulation layer due to the stress, and cold insulation performance is decreased.
As described above, in the under-ground tank, it is possible to construct the cold insulation layer, which is disposed between the foundation slab and the inner tank, with a sufficient thickness. Accordingly, a decrease in the cold insulation performance because of the stress generated due to the linear load does not occur. However, in the above-ground tank, since the thickness of the cold insulation layer is limited, a decrease in the cold insulation performance due to the stress greatly damages the performance of the low-temperature tank itself.
The present disclosure is made in consideration of the above-described circumstances, and an object thereof is to provide an above-ground low-temperature tank capable of providing a pump pit while preventing a decrease in cold insulation performance.